Investment casting of titanium and titanium alloys and similar reactive metals in ceramic molds is made difficult by the metal's high affinity for elements such oxygen, nitrogen, and carbon. At elevated temperatures, titanium and its alloys can react with the mold facecoat that typically comprises a ceramic oxide. For example, at elevated temperatures during investment casting in a ceramic investment shell mold having a ceramic oxide facecoat, such as zirconia, a titanium alloy such as Ti--6Al--4V will react with the ceramic oxide to form a brittle, oxygen-enriched surface layer, known as alpha case, that adversely affects mechanical properties of the casting and that is removed by a post-casting chemical milling operation as described, for example, in Lassow et al. U.S. Pat. No. 4,703,806.
Moreover, ceramic oxide particles originating from the mold facecoat can become incorporated in the casting below the alpha case layer as sub-surface inclusions by virtue of interaction between the reactive melt and the mold facecoat as well as mechanical spallation of the mold facecoat during the casting operation. The sub-surface oxide inclusions are not visible upon visual inspection of the casting, even after chemical milling.
The manufacture of titanium based structural airframe components by investment casting of titanium and its alloys in ceramic investment shell molds poses problems from the standpoint that the castings should be cast to near net shape so as to require only a chemical milling operation to remove any alpha case present on the casting. However, any sub-surface ceramic inclusions located below the alpha case in the casting are not removed by the chemical milling operation and further are not visible upon visual inspection of the casting. There thus is a need in the art for a method of making such structural airframe components by investment casting of titanium and its alloys in ceramic investment shell molds in a manner that enhances detectability of any sub-surface ceramic inclusions that may be present below exterior surfaces of the casting.
An object of the present invention is to provide a method of making castings, such as for example, structural airframe component castings, by casting titanium and its alloys as well as other metals and alloys in contact with a mold facecoat that satisfies this need by providing for ready detectability of sub-surface ceramic inclusions that may be present below the exterior surface of the casting.